WSP04447

• File Number: 8220.101.10.D
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 1/1/1999
• Author: Schmidt/John C.
• Title: Summary & Synthesis of Geomorphic Studies Conducted During the 1996 Controlled Flood in Grand Canyon
• Water Supply Pro - Doc Type: Report/Study

334 SUMMARY AND SYNTHESIS OF GEOMORPHIC STIJDlES

with time. Thus. Ihe daily transported load decreased during
the flood even though discharge was steady. This finding is
consistent with measurements of sediment transport in the
Colorado River system during pre-dam floods ILeopold alld
Maddock,19531.
Rubin el al. [1998] and Topping el al. Ithis volume I
measured coarsening of the suspended load transported past
the Grand Canyon gage between day I and either days 5 or
7 of the flood, depending on the sampler used. They showed
thaL the size or bed sediment correspondingly coarsened
with time and that eddy deposits between river miles 61 and
136 venically coarsened. These dala are impressiye in their
internal consistency. There is disagreement about temporal
Lrends in the size of suspended sand further downstream at
National Canyon and abOUL the mechanism Lha. detennined
those temporal trends [Topping el al.. this yoIume; Smilh,
this volume].
The high rate of suspended-sediment transport early in
the flood implies thai transport rates are highest when the
bed has a large proportion of fines. Thus, it may not be
possible to slOre large amounts of fine sediment on the bed
for periods longer than a few years [Topping er aI., this
volume]. Roods designed LO build beaches may need to be
scheduled soon after tributaries have delivered large loads
of fine sedimenL to the mainsLem, because the concentration
of suspended sediment is likely 10 be highest at those times.

6.2. The Appropriale Duralion of Restoralion Floods

Many of the measurements made during the 1996
controlled flood show that only a few days are necessary to
rework debris fans and to deposit eddy sand bars. Thus, the
objectives of remobilizing coarse channel sediment and
reconstructing fine-grained eddy bars and channel-margin
deposits can be achieved by floods of shorter duration that
the 1996 flood.
Field measurements and modeling studies demonstrate
that deposition rates during the first few days of high
discharge were high downstream rrom the Little Colorado
River [Wiele el al.. this volume]. Rapid rates of channel-bed
adjustment and high deposition rates in eddies had been
documented in flume [Schmidl el al.. 1993] and modeling
studies (J.M. Nelson and R.R. McDonald. U.S. Geological
Survey. wrinen commun.. 1995) using simple channel
geometries.
The area-weighted deposilion in 3 eddies immedialely
downstream from the Little Colorado River declined greatly
between days 0 and 4 (Figure 2a), and the area.weighted
deposition in these eddies was more variable during (he last

3 days of the flood. Area.weighted erosion and deposilion
were calculated as

D, = V derl Ai' and

(I)

E, = VerolAI'

(2)

where D. = area.weighted deposition, E, = area-weighLed
erosion, A, = the total area surveyed in the eddy, V dep = the
volume of deposition, and Vero = the volume of erosion.
These calculations were made from the original survey data
of Andrews e1 al. Ithis volume, referenced electronic data
base] and the outlines of persistent eddies shown by
Schmidl el al. [this volume), rather Ihan lhe eddy oullines
used by Andrews et al. (this volume}. in order to faciiilate
comparison with other investigations. The primary
difference between the boundaries of the eddies in the two
investigations is that those of Andrews el al. [this volume]
extend further into the channel and were estimated from
field observations (Figure 3). The boundaries of Schmidl el
al. [this volume] were computed from a phologrammetric
method.
The field measurements are consistent with the predic-
Lions from models of the evolution of the channel and eddy
bed in response to the high suspended-sediment transport of
the winter 1993 flood [Wiele e1 al.. 1996] and the lower
transport of the 1996 controlled flood. Initial deposition
rates in main-channel pools and in eddies in 4 short reaches
had been high in the Colorado River during the winter flood
of 1993. The sediment-transport conditions during the 1993
flood were similar to high-concentration floods that had
occurred prior to construction of Glen Canyon Dam [DJ.
Topping, cited by Wiele el al., 1996]. Deposition and
erosion rates were sufficiently high such lhal channel
bathymetry adjusted to maintain downstream transport of
the large sediment loads within the first 3 days of that flood,
and channel and eddy deposition rates were very low there.
after. The time necessary to achieve a new equilibrium
condition was somewhat longer where the volume of the
channel expansion was large. Wiele el al. [this volume] and
Wiele [1998] showed that channel bed topography also
adjusted quickly during the 1996 controlled flood. Wiele el
al. [this volume] showed that eddy-bar deposition rates
were highest during the first days of the flood, when the
concentration of mainstem suspended sediment was high
and Ihe eddies were empty.
Wirle el al. [this volume] and Wiele I I998J thus showed
that Ihe resuhant topography difrers greatly depending on
the concentration of sediment in transport. These results
confirm the overriding sensitivity of posl-ftood bar form on